Gobiid fishes are distributed in diverse aquatic habitats including marine, brackish, and freshwater environments from tropical to temperate regions, encompassing approximately 1430 diverse species (Sureandiran et al. 2024; Fricke et al. 2025). Among the gobiid fishes, shrimpgobies (genus Tomiyamichthys Smith, 1956) are mainly found in the Indo-Pacific region and often engage in a symbiotic relationship with alpheid shrimp (Randall and Chen 2007). Such symbiotic gobiid species include approximately 150 species in 11 genera (Eschmeyer 2015), Tomiyamichthys gobies include 22 valid species (Sato and Motomura 2025), which are distinguished from other gobiid genera by having half or most of the ventral part of the first gill slit closed by a membrane, well developed longitudinal pattern of sensory-papillae rows on cheek and a uniserial transverse row of sensory-papillae behind the chin (Shibukawa et al. 2005; Hoese et al. 2016). Seven Tomiyamichthys species have been recorded in Japanese waters (Nakabo 2013; Sato and Motomura 2025), but no occurrence reported around Korean waters.

Tomiyamichthys oni (Tomiyama, 1936) was reported firstly as a new species in 1936 (Tomiyama 1936), and several studies on the taxonomic checklist have been conducted thereafter (Allen and Erdmann 2012; Sonoyama et al. 2020; Parenti 2021). Due to limited information on this species, it is currently classified as of Least Concern by the IUCN (Larson et al. 2021). It is distributed in the eastern Indian and western Pacific Oceans, from the Andaman Sea and Malaysia to Papua New Guinea, New Caledonia, Fiji, Palau, the Philippines, and Japan, with depth range of 5–35 m (Karplus and Thompson 2011; Nakabo 2013; Allen 2015; Hoese et al. 2016; Allen et al. 2018). This species is subtropical, with its northernmost distribution recorded at 35°00'N based on current collection and habitat record (GBIF 2026).

Although T. oni was initially observed via underwater visual surveys in the southern coastal waters of Jeju Island, Korea, in early September 2021 (B. J. Kim, pers. observ.), formal documentation integrating taxonomic measurements and DNA barcoding has not been reported from Korean waters. In this study, we report the first occurrence of T. oni in Korean waters based on a single voucher specimen obtained in SCUBA diving from Ulleung Island. The identification of the species was grounded on the morphological characteristics and mitochondrial cytochrome c oxidase subunit I (COI) sequences. Additionally, this study reports the new northernmost distributional record for the species.

One specimen of Tomiyamichthys oni has been collected from the southwestern part of Ulleung Island (37°27'57"N, 130°49'51"E) on 1 March 2025 (Fig. 1). Sampling was conducted via diurnal SCUBA diving surveys at a depth of approximately 25 m on a mixture of sandy and rocky substrates, with a dead specimen recovered. The collected fish were transported to the laboratory, where photographs were taken and the specimens were preserved in 99% ethanol. Meristic counts and body morphometrics were recorded following the method of Randall and Chen (2007). Seven meristic counts and twenty-five body part measurements (to the nearest 0.1 mm) were conducted using digital Vernier calipers and under stereomicroscope. The specimen was then deposited at the National Institute of Fisheries Science, Fisheries Resources Research Center (NIFS_FRRC). The collection site was compared with the historical records of the species registered in the Global Biodiversity Information Facility (GBIF 2026).

To compare molecular data, total genomic DNA was extracted from the muscle tissue using the GeneAll Exgene™ Clinic SV DNA extraction kit (GeneAll, Seoul, Korea). The mitochondrial DNA (mtDNA) cytochrome oxidase subunit I (COI) region was amplified for molecular analysis using the VF2 primer (5'-TCA ACC AAC CAC AAA GAC ATT GGC AC-3') and FishR2 primer (5'-ACT TCA GGG TGA CCG AAG AAT CAG AA-3') (Ward et al. 2005). The PCR profile consisted of initial denaturation at 95°C for 5 min, followed by 35 cycles of denaturation at 94°C for 30 s, annealing at 52°C for 30 s, extension at 72°C for 1 min and a final extension at 72°C for 7 min. DNA sequences were obtained using an ABI 3730XL DNA Analyzer (Applied Biosystems, Inc., Foster City, CA, USA) with the ABI BigDye Terminator Cycle Sequencing Ready Reaction Kit v3.1. The obtained DNA sequences were registered with the National Center for Biotechnology Information (NCBI) under accession number NIFS_FRRC_3 (PX427231). We compared our molecular data with those of the mtDNACOI sequences from other Gobiidae fishes obtained from GenBank (National Center for Biotechnology Information, www.ncbi.nlm.nih.gov). Sequences were aligned using Clustal W (Thompson et al. 1994) in BioEdit, version 7 (Hall 1999). The genetic divergences were calculated using the Kimura 2-parameter (K2P) (Kimura 1980) model with Mega 6 (Tamura et al. 2013). Phylogenetic trees were constructed using the neighbor-joining method (Saitou and Nei 1987) in Mega 6 (Tamura et al. 2013), with confidence assessed, based on 10 000 bootstrap replications.

Family Gobiidae Cuvier, 1816

Genus Tomiyamichthys Smith, 1956: 553 (type species Cryptocentrus oni Tomiyama, 1936, by original designation)

In this study, a single specimen of a shrimpgoby (genus Tomiyamichthys) has been collected from the southwestern waters off Ulleung Island, Korea. The morphological characteristics of the examined specimens closely align with the original description of T. oni (Tomiyama, 1936), specially exhibiting grooved upper margins on the swollen postorbital cheeks; clusters of irregular brown spots covering body; black band on lower part of opercle extending forward to tip of chin. The result of genetic analysis further confirmed that the specimen matched T. oni, exhibiting a 19.1%–24.0% difference from other congeners. Consequently, based on both morphological and molecular analyses, the present species from Ulleung Island, Korea was identified as T. oni which represents the first record in Korean waters. However, the numbers of dorsal- and anal-fin soft rays (12 and 10, respectively) were slightly different from the original record (Tomiyama 1936), which documented 11 soft rays for both dorsal- and anal-fin fin rays (Table 1). The number of anal fin soft rays recorded in this study fell within the range of 10–11 as reported by Allen and Erdmann (2012), but that of dorsal-fin soft rays was consistently different from various previous results, that reported 11 dorsal-fin soft rays (Tomiyama 1936; Masuda et al. 1984; Nakabo 2013). Such a slight difference in fin rays between present and previous studies is likely an intraspecific variation. Nonetheless, the primary distinguishing features between T. oni and its closest congener, T. elliotensis, include the presence of brown spots on the body of T. oni, which are absent in T. elliotensis, and differences in the number of dorsal and anal fin soft rays (11 vs. 10) (Allen et al. 2023). Further morphological studies incorporating specimens from the broader distributional range of this species are necessary to better understand its morphological diversity (Tuset et al. 2018).

Tomiyamichthys oni was initially observed in early September 2021 during SCUBA-based surveys of the subtidal zone of Seopseom (33°N) located in the southern coastal waters of Jeju Island, Korea. These monitoring efforts have been conducted since April 2021 by citizen science organization of ‘the Fish Class Jeju, Korea’. The specimens was observed at a depth of 15 m on a sandy benthic substrate, where it continued to be recorded to cohabit with a symbiotic shrimp at a fixed site monthly until early August 2022 (Fig. 4), prior to the coastal passage of Typhoon Hinnamnor (11th typhoon of 2022) affecting Jeju Island (B. J. Kim, unpublished data). This species primarily inhabits sandy sea bottoms on flat terrain, burrowing into the substrate and forming a symbiotic relationship with shrimp of the genus Alpheus (see Randall and Chen 2007; Allen et al. 2023). In this mutualistic interaction, the snapping shrimp excavates and maintains the burrow, while the fish acts as a sentinel, guarding against potential threats (Randall and Chen 2007). Due to its rapid retreat into burrows upon detecting disturbances (e.g., divers), the collection of specimens proved challenging, and only photographic records were obtained.

Tomiyamichthys oni is primarily distributed in subtropical waters, with a known latitudinal range extending from 35°01'35"N to 21°49'21"S (Froese and Pauly 2026; GBIF 2026). In comparison with previous records, this study confirmed a new northernmost record (37°27'57"N) for the species, suggesting a northward expansion of its distributional range. (Fig. 5). This poleward range extension is likely attributed to climate change-driven ocean warming, as surface water temperatures (SST) in Korean waters have steadily increased over recent decades (Han et al. 2023). In particular, the East Sea, Korea which is influenced by the Tsushima Warm Current, has exhibited a higher warming rate compared to the southern and western waters of the Korean Peninsula; this trend was further exacerbated in 2024 by an intensified intrusion of warm currents, resulting in prolonged periods of elevated sea temperatures (Choi et al. 2025). Globally, numerous studies have also reported that rising sea temperatures and changing ocean currents are causing the poleward expansion of various marine species (Pinsky et al. 2020; Son et al. 2020; Myoung et al. 2023; Hwang et al. 2024). The documentation of T. oni at Ulleung Island may serve as an early indicator of climate-driven ecological changes. However, because this study recovered only a single deceased specimen—lacking evidence of live individuals or the symbiosis characteristic with alpheid shrimp observed off the southern coast of Jeju Island—it remains undetermined whether this result represents an established population or a transient occurrence around Ulleung Island. The successful colonization in a new habitat by range-shifting marine taxa is fundamentally constrained by the thermal sensitivity of their physiological traits and adaptive capacity (Berg and Ellers 2010; Thompson and Fronhofer 2019). Therefore, long-term and continuous latitudinal monitoring is required to determine whether the presence of this species is a transient response to the anomalous thermal conditions in 2024, or a recurring seasonal dynamic governed by overwintering mortality (Eme and Bennett 2008; Figueira et al. 2009; Gajdzik et al. 2021; Osland et al. 2021).

This study provides the first report of T. oni inhabiting the coastal waters of Korea and offers insights into the range expansion of this species in response to ongoing climate change. Based on the distinctive morphological characteristic of a black band below the eyes, a Korean name ‘Geom-eun-tti-nun-se-u-mang-dug’ is proposed for this species. The term “Geom-eun” means black color, “tti” means band or stripe, referring to the distinct band-like markings under the eye; and “nun” means eye. The term “se-u-mang-dug” literally means shrimp goby, reflecting its symbiotic association with shrimp and its taxonomic placement within the gobies. Additionally, our findings confirm that T. oni was collected further north than previously reported distribution ranges, indicating a climate change-induced habitat expansion of this species. Nonetheless, due to the lack of the observation true living status, further research is required to assess how this shift affects the native species inhabiting temperate marine ecosystems. Such studies will serve as valuable resources for understanding interactions between newly introduced species and native marine organisms. In addition, understanding the poleward expansion of subtropical fish species into temperate seas is essential for predicting the future impacts of climate change on marine biodiversity.